FIG. 1 is a diagram of the structure of a conventional digital subscriber line transmission system. In FIG. 1, an NT transmission device (hereinafter referred to as an NT station) 11 located at a subscriber (NT) side and an LT transmission device (hereinafter referred to as an LT office) 12 located at a exchange office (LT) side are connected by means of a transmission line 13 via an interface, the transmission line 13 enabling two-way transmissions. The transmission device 12 at the LT side is connected to a subscriber line exchange 12a, which transmits users' data (B, D and M respectively denote channels) between users in the two ways. More particularly, information having a bit rate of 160 kbps is transferred between the NT station 11 and the LT office 12 via the transmission line 13.
The NT station 11 and the LT office 12, which perform the two-way transmission, carry out, for the purpose of line equalization and echo cancellation, training in advance of reception and transmission.
FIG. 2 is a block of the stations shown in FIG. 1. In FIG. 2, a transmission/reception part 11A at the NT side and a transmission/reception part 12A at the LT side are the same as each other. Transmission data is sent to an echo canceller (EC) 22 and an encoder (ENC) 23 via a transmitter (Tx) 21.
The echo canceller (EC) 22 generates an echo replica of echo of the near-end transmission, and the echo replica is eliminated from the transmission signal at a subtracter 24. Data encoded by the encoder (ENC) 23 is transmitted to the transmission line 13 via a hybrid circuit (H) 25. The hybrid circuit (H) 25 is a circuit establishes an interface between a subscriber line (two wires) and a transmitter/receiver part (four wires), and establishes impedance matching (line equalization) by a balancer (B) 25a.
An analog input signal that is input from the transmission line 13 is sent to the subtracter 24 via the hybrid circuit (H) 25. The echo of the near-end transmission is separated from the input signal at the subtracter 24, and is sent to a receiver 26. In this manner, received data is extracted. The subtracter 24 functions to eliminate the transmit data from the input signal because the two-way communications take place via the transmission line 13 and the echo of the near-end transmission is included in the input signal via the hybrid circuit 25.
The input signal transmitted via the hybrid circuit 25 is sent to a tone signal detection circuit (TD) 27, which detects a tone signal contained in the input signal. The detected tone signal is used for training which is performed at the previous stage of actual data communications.
FIG. 3 is a block diagram of the tone signal detection circuit shown in FIG. 2. In FIG. 3, the tone signal detection circuit 27 an A/D converter 31 for converting an analog input signal into a digital signal, a band-pass filter (BPF) 32 for extracting signals having necessary frequency components, and a level (power) detector 33 for detecting the level (or power) of the signal from the band-pass filter 32.
In practice, the A/D converter 31 of the tone signal detection circuit needs two operational amplifiers, two comparators, and a digital circuit having 1500 gates if the tone signal consists of 14 bits and has a speed of 80 kbauds (kHz). Further, the BPF 32 and the level (power) detector 33 need 1000 gates or more.
FIG. 3 shows the case where the analog input signal is converted into the digital signal. In a case where the tone signal is detected from the analog input signal rather than the digital signal, the A/D converter 31 is not needed, and the BPF 32 and the level (power) detector 33 are analog circuits made up of a large number of comparators and so on.
The above-mentioned digital subscriber line transmission system is configured in conformity to a U. S. standard (ANSI: American National Standards Institute). This U.S. standard standardizes an interface required to perform communications between stations and networks in a digital subscriber line transmission system.
According to the above U.S. standard, a code transmitted via the transmission lines is a 2B1Q code, which is a PAM (Pulse Amplitude Modulation) code having amplitudes of four values and no redundancy.
FIG. 4 is a diagram for explaining the 2B1Q code. As shown in FIG. 4, the 2B1Q code is such that two bits are represented by 4-nary symbols (+3, +1, -1, -3).
FIG. 5 shows an example of a training sequence, and FIG. 6 shows an example of the tone signal.
Referring to FIG. 5, a tone signal TL for training equal to two frames (240 symbols) is transmitted from the LT (office) side shown in part (a) of FIG. 5, and a training tone signal TN for training equal to four frames (480 symbols) is transmitted from the NT (subscriber) side shown in part (b) thereof. That is, a period (4 msec or less) for receiving the tone signal TL for training is provided at the NT (subscriber) side shown in the part (b), the tone signal TN is transmitted upon receipt of the tone signal TL. The training is sent to the NT (subscriber) side after the tone signal TN is received and detected at the LT (office) side.
As shown in FIG. 6, the tone signal is a signal of 10 kHz which is repeated with a period of eight symbols (+3, +3, +3, +3, -3, -3, -3, -3) of 80 k-baud. By sending the tone signal to the other device, the respective devices are informed of the starting of training by each other. In the modems and so on, a sinusoidal wave signal is used as the tone signal.
Normally, supplying of power to parts that are not needed to operate when the training is started before a communication takes place is interrupted in order to reduce power consumed in the digital subscriber line transmission devices.
However, it is impossible to interrupt supplying of power to a circuit for detecting the tone signal because such a circuit is needed to start the training. Furthermore, the tone signal detection circuit 27 shown in FIGS. 2 and 3 needs a large number of circuit parts even when the circuit 27 is of analog type or digital type. As a result, the circuit 27 is very complex and consumes a large quantity of power.